The purpose of this study is to use an experimental form of gene therapy treatment for hemophilia, using a human Factor VIII retrovirus vector [hFVIII(V)]. In gene therapy, a gene is put into cells to provide different instructions from those provided by the genes a person already has. Genes are like "blueprints" which provide detailed instructions to a cell about how to function. People with severe hemophilia are born without a gene, which makes normal FVIII. The goal of gene therapy for hemophilia is to provide a FVIII-producing gene to the body cells so that a person with hemophilia will be able to make his own FVIII. There are two primary goals of this study: 1) to find out how safe and effective human factor VIII vector treatment is for individuals with severe hemophilia A and 2) to measure the amount of Factor VIII which is produced in the body after a given dose of human factor VIII vector in order to determine the best dose for treatment.